Many types of fibers exist and find a variety of applications ranging from domestic garments to the field of space science. Fibers in various forms contribute to the development of industry. For example, fibers can be roughly classified into organic fibers, such as natural fibers obtained from plants and animals, synthetic fibers obtained by synthesizing organic chemical substances, semi-synthetic fibers, and regenerated fibers; and inorganic fibers, such as carbon fibers obtained by carbonizing, for example, an acrylic fiber or pitch as a starting material at a high temperature, glass fibers, metal fibers, and rock fibers.
Among these fibers, in particular, carbon fibers, which are lightweight and have mechanical properties of being very strong and highly elastic, are combined with a matrix resin and used as a fiber reinforced matrix composite.
Carbon fibers, which are typically composed of monofilaments having a diameter of several microns, have low elongation and thus easily fluff due to mechanical friction, etc. and are often difficult to handle. Therefore, carbon fibers are usually sized with a fiber treatment agent and used (see PTL 1 and 2). A fiber treatment agent enhances the bundling properties of carbon fibers, and a sizing treatment using a fiber treatment agent may also improve the physical properties of carbon fibers or carbon fiber bundles. Furthermore, the treatment agent present on the surface of carbon fibers can enhance the compatibility with a matrix resin and increases the adhesion of the interface between the matrix resin and carbon fibers.
Matrix resins that are combined with carbon fibers can be roughly classified into thermosetting resins and thermoplastic resins. When a thermoplastic resin is used as the matrix resin, a high toughness composite material can be easily obtained, compared to the use of a thermosetting resin, with excellent thermal processability as well, thus having a high utilization value.
Examples of fiber treatment agents that enhance compatibility with such a thermoplastic resin include polyamide-containing treatment agents. Carbon fibers treated with a polyamide-containing treatment agent for fibers have excellent compatibility with various thermoplastic resins, such as polyamides, polyesters, polyethylenes, and polycarbonates.
Examples of such polyamide-containing fiber treatment agents include those comprising water-soluble polyamides (see PTL 3, 4, and 5). These treatment agents are not only highly soluble in water but also impart sufficient bundling properties to shape carbon fibers and allow the resulting carbon fiber bundles to be uniformly dispersed in water. Therefore, such treatment agents can be preferably used to uniformly disperse carbon fibers in an inorganic matrix slurry, such as concrete. However, water-soluble polyamides, which originally have the property of being soluble in water, absorb a greater amount of moisture in the air than common water-insoluble polyamides and may impart tackiness to the carbon fiber bundle surface with time, thus impairing the processability of carbon fiber bundles. Further, when carbon fiber bundles are adhered to a matrix resin to form a composite material, delamination tends to occur due to the influence of moisture absorbed by the treatment agent, and mechanical properties, such as strength and flexural elasticity, and durability of the molded product may deteriorate. Accordingly, in applications in which the treatment agent tends to be in contact with water or air, usable ranges may be limited.
To improve the water resistance of such water-soluble polyamides, a method comprising crosslinking a water-soluble polyamide with a curing agent, such as blocked isocyanate, to make the polyamide insoluble in water (see PTL 5), and a method comprising heating until a water-soluble polyamide is self-crosslinked to be insoluble in water (see PTL 6) are known. However, when a crosslinking method is used, a functional group (e.g., a carboxyl, amino, or hydroxyl group), which is present on the carbon fiber interface and which helps to improve the adhesion through a hydrogen bond to the matrix resin, reacts. This reduces the functions of the functional group and may lower the adhesion. The heat treatment method may also thermally degrade polyamides and matrix resins.
It is also known that a water-insoluble polyamide, such as copolymer nylon, is dispersed in the form of particles in water and the resulting aqueous polyamide resin dispersion is used as a fiber treatment agent (see PTL 7).
When such a water-insoluble polyamide resin (water-insoluble polyamide) is formed into an aqueous dispersion and used as a fiber treatment agent, the fiber treatment agent has higher water resistance than a fiber treatment agent comprising a water-soluble polyamide, and thus can be used in applications for which water-soluble polyamides are unsuitable.
Compared to using a water-soluble polyamide resin or like solution as a fiber treatment agent, aqueous polyamide resin dispersion-type fiber treatment agents can more firmly fix the resin on the surface of carbon fibers or in the voids of carbon fiber bundles. This can greatly improve the heat resistance and physical properties of the carbon fibers or carbon fiber bundles. Accordingly, in particular, there has recently been an increasing demand for aqueous dispersion-type fiber treatment agents.
However, compared to using a water-soluble polyamide, the use of a water-insoluble copolymer nylon or like polyamide generally results in slightly poor adhesion to a matrix resin, partly due to the strong crystallinity of the resin. Accordingly, adhesion improvement has been desired.